Fungi in corals: symbiosis or disease? Interaction between polyps and fungi causes pearl-like skeleton biomineralization

The skeleton of live coral Porites lobata is regularly bored by euendolithic algae (mostly Ostreobium quekettii) and fungi, both commonly extending up to the very tips of newly produced skeletal spines. The live polyp tissue of P. lobata occupies a 4 to 5 mm thick surface layer of the corallum, within which new skeletal material is deposited. Thus, the endoliths do not constitute a separate zone beneath the live polyps; rather, the polyp tissue and populations of endolithic algae and fungi of significant densities co-exist and interact within the same layer. Aragonitic, hemispherical to conical outgrowths protruding from the walls of structural pores were observed in skeletons of P. lobata from the barrier reef of Moorea Island, near Tahiti, French Polynesia. These protrusions were always associated with endolithic fungal hyphae attempting to exit from the skeleton into the space occupied by polyps. The polyps responded to such intrusions in a manner similar to the response of mollusks to foreign bodies: by local deposition of dense skeletal material. As the fungus continued to penetrate through this repair deposit, new layers of aragonite were added by the polyp, contributing to the growth of the protrusions. Fungal hyphae rarely entered the pore spaces while these were still occupied by coral polyps. More often, the polyps escaped the fungus by moving upward, as a part of their normal growth rhythm, evacuating the previously occupied skeletal pores. Deprived of resistance, fungal hyphae penetrated through the cones and exited into emptied pore spaces. The conical structures were affected by diagenesis differently than the intact skeletal carbonate. Both skeletal carbonate and repair carbonate were subject to syntaxial diagenetic crystal growth, but they resulted in cements with disparate crystal sizes. Septate fungal hyphae are common in coral skeletons as euendoliths, as cryptoendoliths in structural voids, and as endophytes inside filaments of endolithic algae. They were also found inside soft coral tissue. Fungi may be opportunistic pathogens in corals under environmental stress. Their activity, recorded and preserved in the coral skeleton, provides information on changes in past conditions of coral growth.